1. Field of the Invention
This invention relates generally to an improved process for making activated carbon using a fluidized bed technique for conditioning coal particles preliminary to carbonization and final activation. According to this invention, the process employs an oxidation treatment of prepared coal particles in a reactor having means for maintaining close control of the oxidation temperature and average residence time.
2. Description of the Prior Art
Numerous techniques have been heretofore proposed for making activated carbon. As will be appreciated by those skilled in this area of technology, the particular route taken depends -- to a great extent -- on the nature of the starting material, the end product desired in relation to its industrial application. Typical starting materials include coconut shells, used cooking liquors from paper mills and coal. Thus, where coal has been used as a carbon source, it is the usual practice to prepare the coal by one or more of the following conventional steps, crushing, washing, compaching, and sizing. Thereafter, the coal particles are heated to an elevated temperature wherein the volatile matter is substantially driven off.
One suggested approach to making activated carbon resides in the use of an upright retort such as described in U.S. Pat. Nos. 2,536,782 and 2,536,105, both being issued to K. B. Stuart on Jan. 2, 1951. The Stuart apparatus, directed only to carbonization and activation, comprises an inlet at the top of the retort for ingress of carbonaceous material and an outlet at the bottom for egress of activated carbon, the retort being divided into an upper carbonizing chamber and a lower activating chamber with a partition disposed between the two chambers with at least one opening therein for the passage downwardly of char from the upper chamber to the lower chamber and for the passage upwardly of gases from the lower chamber to the upper chamber.
Another technique for activation resides in the use of a kiln, wherein a bed of carbonaceous material is continually agitated at elevated temperatures by mechanical stirrers or by providing a rotating kiln to expose the coal to the action of hot reactive gases. This technique is referred to in the text, Activated Carbon by J. W. Hassler, Chemical Publishing Co., Inc., NY 1963, at page 181.
After an extensive period of investigation, it was found advantageous to employ one or more fluidized bed reactors for oxidation under controlled temperature and controlled environmental conditions to make activated carbon from bituminous coal. Bituminous coal particles become plastic-like and stick together when heated to 800.degree.F., or thereabouts depending on the nature of the coal used, its particle size, etc. This "agglomerative" effect -- as it is commonly called -- is caused for the most part by the presence of tars and other volatiles present in the raw coal. This undesirable agglomerative characteristic is particularly troublesome where fluidized bed reactors are employed. As particles clump and grow larger, the fluid reactor can become plugged and must be cleaned. Moreover, as the particles grow larger, it becomes more difficult to maintain the particles in a fluidized condition which is necessary for efficient reaction. To avoid this particular problem, various suggestions have been made. For example, in U.S. Pat. No. 3,047,472 to Gorin, crushed coal is oxidized first at about 600.degree.F., followed by a second oxidation at a temperature in excess of 850.degree.F. The U.S. Pat. No. 3,076,751 to Minet also discloses a process for making char and recovering volatiles from coal. In this process, however, it is noted that an inert gas is used in a first reactor maintained at a temperature which can be as high as 1,600.degree.F. The patents to Eddinger et al., U.S. Pat. Nos. 3,375,175 and 3,565,766, disclose multi-stage fluidized bed processes for pyrolyzing bituminous coal to obtain increased yields of oils and tars. Inert gas is employed as the fluidizing medium in both the initial pretreatment and higher temperature pyrolysis with the oxidizing fluidizing medium employed in the latter partial gasification stage where the temperature is at least 1,500.degree.F.
From the foregoing brief description of pertinent prior art methods, it is seen that fluidization treatment of coal has been utilized, but not in the context of making activated carbon having predetermined tailored properties. Moreover, although oxidation of fluidized carbonaceous particles has been suggested, as a practical matter, this approach has met with difficulty, at least up until this invention.
Various suggestions have been made for controlling the oxidation reaction temperature, principally for pretreatment prior to gasification process wherein there is minimum oxidation. These include:
1. varying the oxygen concentration of the fluidizing gas,
2. recycle of a portion of the oxidized product, and
3. immersion of cooling coils in the fluid bed.
Although the above-mentioned techniques provide some measure of temperature control, each suffers from disadvantages from a technical, as well as, commercial view point.
For instance, where the oxygen concentration is varied, a high degree of expensive, sophisticated analytical instrumentation is required. Moreover, this type of control tends to lag and does not have the best response to maintain proper, steady state conditions.
Where a portion of the oxidized product is withdrawn, cooled, and then returned to the fluidized bed to control the reaction temperature, as described in U.S. Pat. No. 2,560,478 to B. C. Roetheli, the oxidized coal recycle rate would have to be about 10 times larger than the feed rate in order to absorb the excess heat. Such high product recycle rate would result in a lower oxidized product yield because of attrition losses, particularly since the particle hardness is relatively low during the oxidation step.
Where the cooling coils are immersed in the fluidized bed to control the reaction temperature in the oxidation step, a considerably reduced throughput is obtained. Moreover, coils inside the bed would result in higher attrition losses thus decreasing the oxidized product yield. Cooling coils would also give wider variations in steady state operating temperature and slower response to upsets in operating parameter because of the lag time for heat transfer inherent in cooling coils.
Thus, it will be apparent from the description which follows that the process according to the present invention has successfully overcome the disadvantages of the prior art approaches in a most efficient and economic manner.